Radiation therapy involves medical procedures that selectively expose certain areas of a human body, such as cancerous tumors, to high doses of radiation. The intent of the radiation therapy is to irradiate the targeted biological tissue such that the harmful tissue is destroyed. In certain types of radiotherapy, the irradiation volume can be restricted to the size and shape of the tumor or targeted tissue region to avoid inflicting unnecessary radiation damage to healthy tissue. For example, conformal therapy is a radiotherapy technique that is often employed to optimize dose distribution by conforming the treatment volume more closely to the targeted tumor.
Normal physiological movement represents a limitation in the clinical planning and delivery of conventional radiotherapy and conformal therapy. Normal physiological movement, such as respiration or heart motion, can cause a positional movement of the tumor or tissue region undergoing irradiation. If the radiation beam has been shaped to conform the treatment volume to the exact dimensions of a tumor, then movement of that tumor during treatment could result in the radiation beam not being sufficiently sized or shaped to fully cover the targeted tumoral tissue.
To address this problem, the size and/or shape of the radiation beam can be expanded by a “movement margin” (i.e., the predicted movement distance in any direction of the targeted tumor) to maintain full irradiation of the targeted tissue. The drawback to this approach is that this increased irradiation volume results in radiation being applied to otherwise healthy tissue that is located within the area of the expanded volume. In other words, motion during treatment necessitates the application of a radiation field of an expanded size that could negatively affect an unacceptably large volume of normal tissue surrounding the targeted treatment volume.
Another approach to this problem involves physiological gating of the radiation beam during treatment, with the gating signal synchronized to the movement of the patient's body. In this approach, instruments are utilized to measure the physiological state of the patient with reference to the particular physiological movement being examined. For example, respiration has been shown to cause movements in the position of a lung tumor in a patient's body. If radiotherapy is being applied to the lung tumor, then a position sensor or a strain gauge can be attached to the patient to measure the patient's respiration cycle. The radiation beam can be gated based upon certain threshold points within the measured respiratory cycle, such that the radiation beam is disengaged during periods in the respiration cycle that correspond to excessive movement of the lung tumor.
Applicant of the subject application believes that a new method and system for determining a physiological state, such as a breathing amplitude or a breathing phase, of a patient may be desirable.